1. Field of the Invention
The present invention relates to an image data processing method and apparatus. More particularly, the present invention relates to an image data processing method and apparatus that converts an image resolution of image data into a higher one. The image data processing method and apparatus can be applied to an optical printer such as a laser printer, an electrographic image forming apparatus such as a digital photocopier, a plain paper facsimile machine, an image display device, etc.
2. Discussion of the Background
An image formed by an image forming device and image display device according to a digital image signal or image data is produced as an aggregation of a plurality of discrete dots or picture elements (pixels). Such dots or pixels are generally allocated as a matrix state, and therefore such an image is sometimes referred as a dot matrix image or a bit mapped image.
As one characteristic of bit mapped images, image resolution is defined and measured as dots per inch (dpi), as an example. Image resolution generally varies depending on image input devices, such as an image scanner, and also varies depending on image output devices, such as a laser printer and a monitor display. Further, application programs implemented in computers, such as a word processing program, a drawing program, a computer aided design (CAD) program, a spread sheet program, an e-mail program, a database program, etc., generate images in various image resolutions. Those images can also be categorized as input images or original images for image output devices.
Therefore, image resolution of an original image generated by an image input device or an application program and image resolution of an image output device are sometimes different from each other. Therefore, in such cases, the image resolution of the input image is converted into another image resolution, i.e., image resolution of the image output device, to reproduce an image of a preferable size.
A higher resolution image has more dots than a lower resolution image per unit area; therefore the higher resolution image has greater data than the lower resolution image for the same size images.
Each dot of a bit mapped image has characteristics such as location coordinates, intensity, color, and size. Each characteristic is independent of the other characteristics and can be viewed as an independent dimension. For example, regarding sizes or intensities of dots, bit mapped images are categorized into bi-level (binary) images such as text and character strings and line images, and multi-level (gray scale) images such as pictures and photographs. Even a single page image may include both binary images and gray scale images.
Image components of an analog image may be continuous in any orientation, while those of a bit mapped image must proceed in orthogonal, incremental steps. This constraint results in distortion in the bit map representation of an analog image. For example, diagonal lines and boundaries between different regions of printer dots produce jagged steps or a staircase distortion, which is quite visible to the human eye. The jagged steps or staircase distortion are also referred as a jagged image.
Either an image reduction by a thinning out operation of dots or an image enlargement by a dot embedding operation often causes such jagged steps. Similarly, when a high resolution image is converted into a low resolution image, the resolution conversion may cause jagged steps or staircase distortion as well.
Methods for reducing or correcting for jagged steps or staircase distortion are known. As an example, U.S. Pat. No. 4,544,922 describes that an original image is first converted into an output image having a higher resolution than that of the original image, then image outlines, such as diagonal lines and boundaries between different regions of printer dots, are processed by a smoothing operation, a rounding operation, an embedding operation for hollows of steps, etc.
As another example, Japanese Laid-Open Patent Application No. 2-112966 describes a jagged image correction method. The method provides a dot pattern sampling window for sampling dots in a focusing region, i.e., dots including a target dot, i.e., a dot to be processed at the moment, and a plurality of surrounding dots. Then, the sampled dot pattern is compared with a plurality of matching dot patterns or templates. Then, the target dot is uniquely processed associated with a coincident matching dot pattern.
As still another example, Japanese Laid Open Patent No. 5-207282 describes an image improving method that corrects boundaries between a black dot region and a white dot region of a bit mapped image. The method provides a recognition step of recognizing shapes of boundaries between a black dot region and a white dot region of the bit mapped image, and a converting step of converting the recognized shapes of boundaries into code information composed by plural bits. The method then determines whether each dot should be corrected based on at least a part of the code information, and corrects the correction required dot according to the code information. The method reduces data size for the correction to be stored in advance in a memory. Further, the method determines the required correction dot for correction data in a short time, only by using a relatively simple determining operation and other operations that can be executed by, e.g., a microprocessor.
Japanese Laid Open Patent No. 5-207282 further describes an image data processing apparatus that executes the above-described image improving method. The apparatus provides a dot pattern sampling window for sampling a target dot and a plurality of surrounding dots. The apparatus also provides a pattern recognition device for recognizing shapes of boundaries between a black dot region and a white dot region of the bit mapped image based on the sampled dot pattern and generates plural bit code information representing the recognized shape.
The apparatus further provides a determining device that determines whether the target dot should be corrected based on at least a part of the generated code information, and a correction data memory to store correction data therein in advance. The address input to the correction data memory is connected to the generated code information, and thereby the correction data memory is accessed to output the stored correction data for the correction required target dot according to the generated code information.
The above-described image data processing method and apparatus do not require storing all templates corresponding to all dot distribution patterns characterized for determining correction required dots. Thus, the image data processing method and apparatus can achieve the determining process of a correction required dot and correction data thereof in a short time according to the generated code information.
As a further example, Japanese Laid Open Patent No. 7-087321 describes an image data processing method that can improve the above-described method of Japanese Laid Open Patent No. 5-207282. The image data processing method performs, by steps (a)-(d) below, an image enlarging operation and an image resolution converting operation as well as the above-described jagged image correcting operation by adding optional information regarding a main-scanning direction X and a sub-scanning direction Y to the above-described code information.
(a) The method first multiplies dots of an original bit mapped image to the main-scanning direction X and to the sub-scanning direction Y such that linearly aligned dots in the main-scanning direction X of the original bit mapped image are multiplied by a positive integer Dr adjoining each other in the sub-scanning direction Y.
(b) The method then recognizes shapes of boundaries between a black dot region and a white dot region in a region including a target dot and surrounding dots in the original bit mapped image, and then generates code information characterized by the shapes of boundaries according to the recognition. The method repeats the above steps for each dot in the original image data one after the other.
(c) Following that, the method counts the multiplied dots at an identical location in sub-scanning direction Y as a count A[15:16] and initializes the count A[15:16] to zero when the count A[15:16] reaches the positive integer Dr. The method repeats the counting and initializing process.
(d) The method then outputs dot data corresponding to the code information and the count A[15:16] to the multiplied dot.
The above-described steps (a) to (d) simultaneously perform an image enlargement in the sub-scanning direction Y or an image resolution conversion and a jagged image correction.
As an example, an image resolution conversion into a lower resolution and an image size conversion into a reduced image are performed by a thinning out operation of dots in an original image. For the thinning out operation, the original image data is once stored in a memory, and a reading operation for the stored data is controlled such that a reading operation for specific dot data is skipped. Conversely, an image resolution conversion into a higher resolution and an image size conversion into an enlarged image are performed by an embedding operation of dots into the original image. For the embedding operation, the original image data is once stored in a memory, and a reading operation for the stored data is controlled such that a multiple reading operation for specific identical data of dots is executed.
When an image resolution of original image data and an image resolution of an image output device, such as a printer or a display device, are fixed values, an image reduction and enlargement are relatively simply performed by the above described dot thinning out and dot embedding operations. However, image resolutions of original images have been diversifying these days, and many printers provide a plurality of resolutions to be selected as well. As recognized by the present inventor, an image data conversion for satisfying various image resolutions of original images, a required image resolution of an image output device, and a required magnification ratio has become burdensome.